The invention relates to electronic filtering systems and methods, and more particularly, to calibration or adjustment of the cutoff frequency in such systems.
Electronic filtering systems are used in wireless communication systems, for example, and especially in cellular mobile telephones whose reception and transmission units incorporate filtering devices. In the reception circuits of cellular mobile telephones, after conversion of the high-frequency analog signal into a low-frequency signal, it is important to filter the low-frequency signal to only retain the useful information. In direct conversion receivers, this filtering is carried out by low-pass filters. Now, it is important to know the cutoff frequency with good accuracy. Also, in integrated systems, this cutoff frequency may vary by up to 30%, based upon the manufacturing process and the operating temperature.
This may then result, during the operation of the telephone, in a loss of the useful signal if the cutoff frequency decreases too greatly, or else in useful signal degradation due to poor rejection of jammers if the cutoff frequency increases too greatly. This is the reason why it is necessary to calibrate the filter, i.e. alter the cutoff frequency of the filter to a known value, in this instance the theoretical cutoff frequency, with greater accuracy.
At present, to perform this calibration, it is possible to use a phase-locked loop whose oscillator is embodied with elements analogous to those of the filter to be calibrated, in particular a resistive/capacitive network. The calibration of the filter is then performed by calibrating the oscillator, and by applying the same correction to the oscillator and to the filter, for example by switching some of the capacitors of the resistive/capacitive network. Now, there is always a matching error between the external phase-locked loop and the filter to be calibrated.
Consequently, the correction determined with regard to the phase-locked loop external to the filter, and applied to the filter itself, is not exact by reason of this matching error. Moreover, the presence of this matching error leaves very little margin for inaccuracy in the technological embodiment of the remainder of the integrated circuit, this being especially penalizing when the cutoff frequency has to be altered with a small tolerance, slightly greater than the matching error. Finally, the presence of an external phase-locked loop for the calibration increases the surface area of the integrated circuit, this having an impact on the cost of embodiment and on the overall size.
An object of the invention is to provide a filtering system whose calibration is not marred by any matching error. An object of the invention is also to provide a filtering system whose calibration constraint requires only a very small increase in the surface area of silicon.
These objects and others are provided by a method of controlling the operation of a monotonic-phase filtering device having a theoretical cutoff frequency. A monotonic-phase filtering device is, for example, a high-pass or a low-pass filter. According to the invention, this method of control comprises a calibration phase in which the filtering device is operated as an oscillator, the frequency of oscillation of the filtering device is determined, and the characteristics of the filtering device are corrected with respect to the determined oscillation frequency and to a pre-established relation between the frequency of oscillation and the theoretical cutoff frequency, in such a way as to confer upon the filtering device a cutoff frequency equal to the theoretical cutoff frequency to within a tolerance.
Moreover, the method of control comprises a working phase in which the filtering device carries out its filtering function. In other words, the invention provides for the use of the filtering device itself as an oscillator. Thus, no external device is used for calibration, and consequently any matching error between such an external device and the filtering device itself is eliminated.
When the filtering device has an order greater than or equal to 3, the filtering device is operated as an oscillator by providing a feedback loop to itself with a phase inversion. Specifically, when the order is greater than or equal to 3, the variation in the phase of the filter as a function of frequency is such that there is a frequency value for which the phase of the filter is equal to +180xc2x0 or xe2x88x92180xc2x0, this permitting its oscillation. Conversely, when the filtering device has an order less than 3, for example an order 2, the value of 180xc2x0 or of xe2x88x92180xc2x0 can never be attained for whatever value of frequency. Consequently, to allow the oscillation of the filtering device, and consequently its calibration, an identical auxiliary filtering device is then advantageously connected in series with the filtering device. Also, the assembly formed by the two filtering devices is operated as an oscillator by feeding this assembly back to itself with phase inversion. This makes it possible to calibrate the filtering device.
When the input of the filtering device is virtual ground, i.e. when the filtering device is for example formed of an operational amplifier, the feedback is advantageously driven with a current injected onto the virtual ground of the filtering device, to alter the amplitude of the oscillations. Thus, the value of the current will advantageously be chosen in such a way that the filter works in the linear operating zone. When the cutoff frequency of the filtering device is defined by a resistive/capacitive network, the characteristics of the filtering device are advantageously corrected by modifying the capacitive value of the resistive/capacitive network.
An aspect of the invention is also an electronic filtering system, including a monotonic-phase filtering device having a theoretical cutoff frequency, and a first controllable means/unit connected to the filtering device and capable of being activated or deactivated in response to a control signal, in such a way as to operate the filtering device as an oscillator when they are activated. The filtering device carries out its filtering function when the first means/unit is deactivated. The system also includes measurement means/unit to determine the frequency of oscillation of the filtering device, correction means/unit to correct the characteristics of the filtering device with respect to the determined oscillation frequency and to a pre-established relation between the frequency of oscillation and the theoretical cutoff frequency, in such a way as to confer upon the filtering device a cutoff frequency equal to the theoretical cutoff frequency to within a tolerance, and a control means/unit to deliver the control signal.
According to an embodiment in which the filtering device has an order greater than or equal to 3, the first means/unit comprises a stage for feedback with phase inversion connected between the output and the input of the filtering device.
According to an embodiment in which the filtering device has an order less than 3, the first means/unit comprises a stage for feedback with phase inversion connected between the output and the input of the filtering device, this feedback stage comprising an auxiliary filtering device identical to the filtering device, and the two filtering devices being connected in series.
According to an embodiment of the invention, the input of the filtering device is a virtual ground and the feedback stage comprises means/unit to drive the feedback with a current in such a way as to alter the amplitude of the oscillations.
According to an embodiment, the filtering device exhibits a differential structure and comprises a differential operational amplifier, and in that the drive means/unit comprises a differential pair of transistors, for example, field-effect transistors with insulated gates whose respective sources or emitters are linked, whose respective gates or bases are linked to the two outputs of the differential amplifier, and whose respective drains or collectors are linked to the two inputs of the differential amplifier, and a current source capable of being activated or deactivated in response to the control signal, and connected to the sources of the two transistors.
The invention is also directed to a remote terminal of a wireless communication system, for example a cellular mobile telephone, incorporating a filtering system as defined herein.